1. Field of Invention
This invention relates to photopolymerizable compositions, particularly to photopolymerizable printing plate compositions, and more particularly to printing plate compositions which are highly sensitive to visible light and have good shelf-life.
2. Information Disclosure Statement
Photopolymerizable compositions have been used in photosensitive elements, such as printing plates, for a number of years. Most representative of the prior art are compositions disclosed in U.S. Pat. Nos. 3,218,167 and 3,887,450. These compositions are relatively slow, however, and must be exposed in the absence of oxygen. Radiation sensitive elements which are oxygen insensitive have been produced by overcoating the elements with an oxygen barrier layer as disclosed in U.S. Pat No. 3,895,949. These elements nevertheless remain relatively slow and such compositions have been satisfactory only where contact printing of the plate and relatively long time exposures can be tolerated.
Photopolymerizable compositions with higher sensitivity are required for applications such as projection printing plates and laser addressable printing plates. For these applications and particularly for laser addressable printing plates, high speed is essential to keep the exposure time short and also to be able to use low intensity lasers, which are of low cost and are more reliable than the high intensity lasers. For these reasons, various efforts have been carried out to improve the sensitivity of photopolymerizable compositions. Some of the representative patents which disclose high speed photopolymerizable compositions are U.S. Pat. Nos. 4,228,232 (3M); 4,594,310 (Mitsubishi Chem Ind KK); 4,259,432 (Fuji Photo Film); 4,162,162 , 3,871,885 and 4,555,473 (Dupont) ; 4,147,552 (Eastman Kodak) and European Patents No. 109,291 (Fuji Photo Film) and 196,561 (Nippon Oils and Fats) and Belgium Patent No. BE 897,694 (Nippon Oils and Fats).
Usually, however, improved sensitivity has been found to be accompanied by poor stability in storage (loss of sensitivity and/or fog generation). For example, the so called "camera speed" photopolymerizable composition described by Bell and Howell ("Unconventional Imaging Process," E. Brinkman, et al, Focal Press New York, 1978, p. 85; "Imaging Systems," K. I. Jacobson et al, John Willey and Sons, 1982, p. 241) has very high photo speed but has poor shelf-life stability.
To improve the storage stability of photopolymerizable compositions, several thermal polymerization inhibitors have been used. These thermal polymerization inhibitors are well known in the art and are described in many patents and literature, such as, U.S. Pat. Nos. 4,168,982; 4,361,640; 4,198,242; 4,148,658. Although the thermal polymerization inhibitors increase the storage stability of the photopolymerizable compositions, one major drawback of using many of these compounds is the decreased photosensitivity imparted by these compounds. This decreased sensitivity is a major hindrance to the potential applications requiring high sensitivity, such as projection exposure imaging and laser imaging.
It is quite well known in the art that the presence of oxygen inhibits free radical polymerization and for this reason the amount of oxygen should be minimized during exposure to obtain a high speed photopolymerizable system. U.S. Pat. No. 3,458,311 teaches the utility of overcoating the photopolymerizable composition with an oxygen barrier top coat layer. A typical oxygen barrier top coat layer is polyvinyl alcohol or gelatin and the layer is usually coated from aqueous solutions on top of the polymerizable light sensitive layer. After exposure, these barrier layers are removed by washing with aqueous solutions, during or before development.
The oxygen barrier layer may contain other additives, such as polyvinlypyrrolidone/vinyl acetate copolymer, discrete inert particles or beads such as polytrimethylolpropane trimethacrylate (U.S. Pat. No. 4,599,299). Strippable oxygen barrier layers are also known in the prior art. U.S. Pat. Nos. 4,454,218, 3,652,275, and 4,148,658 teach lamination of a polyethylene terephthalate film support to the photopolymerizable layer. After exposure the cover sheet is removed.
Trialkylamines, arylalkylamines including p-substituted-N-disubstituted anilines are well known in the prior art. These compounds are known to act as photopolymerization accelerators. U.S. Pat. No. 4,366,228 lists p-substituted-N-disubstituted anilines as photoseveral activators with ketocoumarin compounds. These compounds also act as activators with other keto compounds, such as thioxanthones, fluorenones ("Polymerization of Surface Coatings," C. G. Roffey, Wiley Interscience Publication, 1982), and 1,2-dicarbonyl compounds (U.S. Pat. No. 3,756,827 Re 28,789; 4,072,424).
Many primary amines and some hindered secondary amines are known to be thermal polymerization inhibitors and photo-stabilizers. Napthylamines and phenothiazine are representative examples of thermal inhibitors (U.S. Pat. No. 4,361,640), whereas hindered piperidines are examples of photo-stabilizers for polymers.
Other suitable thermal polymerization inhibitors known in the art are p-methoxyphenol, 2,6-di-tert-butyl-p-cresol, hydroquinone, alkyl and aryl substituted hydroquinones, tert-butyl-catechol, pyrogallol, .beta.-napthol, p-benzoquinone, p-toluquinone, chloranil, nitrobenzene, dinitrobenzene, dicyclopenta-dienyliron, thiazine dyes, e.g. thionine, thionine blue G, methylene blue B, toluidine blue O and 1,4,4-trimethyl-2,5- diazabicyclo [3,2,2] non-2-ene-N,N-dioxide (U.S. Pat. No. 4,361,640).